keyof
keyof用于获取对象的key
type Point = {
x: number;
y: number;
}
type KPoint = keyof Point; // x | y
typeof
const point = {
a : 1,
b: 'test'
}
type P = keyof typeof point; // type '"a" || "b"'
const coordinate: P = 'z' // Type '"z"' is not assignable to type '"a" | "b"'.
